1. Field of the Invention
The present invention relates to electrical transformers and, more particularly, to an electrical transformer having improved short circuit withstand capability through reduced zero sequence current.
2. Description of the Related Art
A conventional two core phase angle regulating transformer consist of an excitor unit and a series unit mounted on separate cores. The excitor unit establishes flux which is transferred to the series unit. The amount of flux transferred from the excitor unit to the series unit determines the phase shift in the series unit. Typically, a two core phase angle regulating transformer is designed for about 50% of total impedance in the excitor unit and 50% in the series unit, so as to balance the physical size of the two units. Both units are connected by cables running in a throat between the two units. The zero sequence reactance of this type of design is a constant. The delta connected secondary winding of the series unit prevents the flow of zero sequence current to the excitor unit. As a result of this, the zero sequence reactance of the phase angle regulator is approximately the zero sequence reactance of the series unit.
During a single line to ground fault, zero sequence current will not flow into the excitor unit and the series unit must be designed to take the full zero sequence current. Since the series unit is designed for 50% of the total phase angle regulator impedance, it should be designed for the short circuit current corresponding to this low impedance. In addition to this, the insulation characteristics of the delta connected secondary winding in the series unit will ordinarily require a 5-leg core design for the series unit, on large phase angle regulating transformers. In order to keep the physical size of the two units the same, the excitor unit will also be designed on a 5-leg core.
A five leg core, however, has three main legs with coils and two outer legs without coils. Accordingly, it has a higher sound level, higher losses, and it uses more material. The tank for housing the transformer must necessarily be longer, and an increased amount of oil is needed to insulate and cool the transformer. The cost of such a transformer is correspondingly higher than for a three leg transformer.